Python Language List Accessing list values
Example
Python lists are zero-indexed, and act like arrays in other languages.
lst = [1, 2, 3, 4]
lst[0] # 1
lst[1] # 2
Attempting to access an index outside the bounds of the list will raise an IndexError.
lst[4] # IndexError: list index out of range
Negative indices are interpreted as counting from the end of the list.
lst[-1] # 4
lst[-2] # 3
lst[-5] # IndexError: list index out of range
This is functionally equivalent to
lst[len(lst)-1] # 4
Lists allow to use slice notation as lst[start:end:step]. The output of the slice notation is a new list containing elements from index start to end-1. If options are omitted start defaults to beginning of list, end to end of list and step to 1:
lst[1:] # [2, 3, 4]
lst[:3] # [1, 2, 3]
lst[::2] # [1, 3]
lst[::-1] # [4, 3, 2, 1]
lst[-1:0:-1] # [4, 3, 2]
lst[5:8] # [] since starting index is greater than length of lst, returns empty list
lst[1:10] # [2, 3, 4] same as omitting ending index
With this in mind, you can print a reversed version of the list by calling
lst[::-1] # [4, 3, 2, 1]
When using step lengths of negative amounts, the starting index has to be greater than the ending index otherwise the result will be an empty list.
lst[3:1:-1] # [4, 3]
Using negative step indices are equivalent to the following code:
reversed(lst)[0:2] # 0 = 1 -1
# 2 = 3 -1
The indices used are 1 less than those used in negative indexing and are reversed.
Advanced slicing
When lists are sliced the __getitem__() method of the list object is called, with a slice object. Python has a builtin slice method to generate slice objects. We can use this to store a slice and reuse it later like so,
data = 'chandan purohit 22 2000' #assuming data fields of fixed length
name_slice = slice(0,19)
age_slice = slice(19,21)
salary_slice = slice(22,None)
#now we can have more readable slices
print(data[name_slice]) #chandan purohit
print(data[age_slice]) #'22'
print(data[salary_slice]) #'2000'
This can be of great use by providing slicing functionality to our objects by overriding __getitem__ in our class.
